The “Controller Area Network” (CAN), as well as an extension of the CAN, referred to as a “Time Triggered CAN” (TTCAN), are known, for example, from German Published Patent Application No. 100 00 305. The media access control method used by the CAN is based on a bitwise arbitration. Bitwise arbitration allows a plurality of subscriber stations to simultaneously transmit data over the channel of the bus system, without any interference to the data transmission in the process. In addition, when transmitting a bit over the channel, the subscriber stations can determine the logic state (0 or 1) of the channel. If a value of the transmitted bit does not correspond to the ascertained logic state of the channel, the subscriber station then terminates the access to the channel. In the case of the CAN, the bitwise arbitration is usually performed in an arbitration field within a data frame to be transmitted over the channel. Once a subscriber station has completed transmission of the arbitration field to the channel, it knows that it has exclusive access to the channel. Thus, the end of the arbitration field transmission corresponds to a beginning of an enable interval during which the subscriber station has exclusive use of the channel. In accordance with the protocol specification of the CAN, other subscriber stations are not permitted to access the channel, i.e., transmit data to the channel, until the transmitting subscriber station has transmitted a checksum (CRC field) of the data frame. Thus, an end time point of transmission of the CRC field corresponds to an end of the enable interval.
A nondestructive transmission of the data frame over the channel is accomplished by the bitwise arbitration. Good real-time properties of the CAN are thereby obtained, whereas media access control methods where the data frame transmitted by a subscriber station can be destroyed during the transmission over the channel due to a collision with another data frame transmitted by a different station, are characterized by a distinctly less favorable real-time behavior since the collision and the thereby necessitated re-transmission of the data frame delay the data transmission.
A further improvement in the real-time behavior of the CAN is attained by the TTCAN extension. In accordance with the TTCAN protocol specification, a time window structure is defined which includes a plurality of successive time windows (often referred to as time slots) and which is regularly repeated. In this context, a specific message type and thus a specific subscriber station can have a specific time window assigned thereto, during which messages of this message type can be transmitted. Thus, in the case of the TTCAN, predetermined time windows are provided during which a specific station has exclusive access to the channel of a CAN domain. In the case of the TTCAN, the access to the channel is at least partially coordinated in accordance with the time based multiple access (Time Division Multiple Access, TDMA) principle.
The protocols of the CAN, respectively of the TTCAN extension thereof, are particularly suited for transmitting short messages under real-time conditions. However, if larger data blocks need to be transmitted over a CAN domain, then the relatively low bit rate of the channel is perceived as interference. To ensure the correct functioning of the bitwise arbitration, a bit transmission requires that a minimum duration be observed that is dependent, in particular, on the extent of the bus system and the signal propagation speed on the channel. Thus, the bit rate cannot be readily increased by reducing the duration of the individual bits.
To nevertheless be able to rapidly transmit a relatively large data block required for programming a control unit via a communication interface actually provided for the connection to a CAN domain, German Published Patent Application 101 53 085 discusses temporarily switching the communication interface for transmitting the data block to a different communication mode in which no bitwise arbitration is performed, so that a relatively high bit rate is possible. In this case, however, it is necessary to interrupt the communication with the CAN protocols for a certain time period. For example, if it is no longer possible to begin the bus system operation in accordance with the CAN protocols due to an error, then the result is bus system failure. Moreover, transmitting a relatively large data block causes a considerable delay in the subsequent transmissions to be undertaken in accordance with the CAN protocols, thereby degrading the real-time properties of the CAN. Thus, using this method to not only program the control unit at the end of a production process for a motor vehicle or the control unit, but also during operation of the motor vehicle, is impractical.